US scientists and activists have called for global prohibition on "germline editing," or the genetic modification of human embryos, saying "there is no justification for, and many arguments against" the looming technology.

Ahead of the International Summit on Human Gene Editing in Washington, DC, around 150 scientists, health practitioners, scholars, and others led by the Center for Genetics and Society (CGS) have grouped together to call for a ban of heritable human genetic modification that could lead to “genetically modified humans” or “designer babies."

Such developments "could irrevocably alter the nature of the human species and society,"they said in an open letter.

Germline editing, during which reproductive cells are modified, stands in contrast to other gene-editing techniques that can be used to alter non-reproductive cells in order to treat impaired human tissue, the group said.

"Gene editing may hold some promise for somatic gene therapy (aimed at treating impaired tissues in a fully formed person)," the letter reads. "However, there is no medical justification for modifying human embryos or gametes in an effort to alter the genes of a future child."

They said human germline editing could result in miscarriage, stillbirth and serious injury to a mother or child, including afflictions that may not materialize until later in a genetically-modified child's life. In addition, such results of germline modification may quash support for "beneficial uses of genetic technologies" in the future, they said.

In conjunction with activist group Friends of the Earth, CGS also released a report on the issue, examining the effects of rapid developments in synthetic biology. The lead author of the study said "avoiding illness, which we can increasingly do, is very different than manufacturing to order."

“Genetic modification of children was recently the stuff of science fiction,” said Pete Shanks, consulting researcher with the CSG. “But now, with new technology, the fantasy could become reality. Once the process begins, there will be no going back. This is a line we must not cross.”

CSG executive director Marcy Darnovsky is scheduled to speak Tuesday at the global summit, convened by the US National Academy of Sciences and similar institutions from China and the United Kingdom. In a statement released prior to the meeting, she said CSG would like the gathering to open a "broader conversation with public voices, including advocates of environmental protection, disability rights, racial justice, reproductive rights and justice, labor, children’s welfare, and others.

“Engineering the genes we pass on to our children and future generations would be highly risky, medically unnecessary, and socially fraught,” Darnovsky said. “There is no good reason to risk a future of genetics haves and have-nots, a world with new forms of inequality, discrimination and conflict.”

Genome editing allows sections of DNA to be precisely removed or replaced using “molecular scissors,” also known as the CRISPR-Cas9 system, during which a scientist would introduce enzymes to attach to the mutated gene.

Germline editing could help treat many inherited diseases if used on the DNA of human sperm, eggs, or embryos, but groups like CSG say the unknown effects of such technology are too dangerous at this point.

This isn't the first call for caution surrounding germline editing. In March, a group of leading biologists called for worldwide boycott on a new technique for editing genomes that results in DNA being altered in a way that could be inherited by future generations.

“You could exert control over human heredity with this technique, and that is why we are raising the issue,” said David Baltimore, a former president of the California Institute of Technology, who was one of the scientists involved in the push.In May, the White House banned germline editing pending more research into the practice. In September, a group of researchers in the UK issued a plea for a wide-ranging ethical debate on the potential pros and cons of such genome editing.